Activated carbon, adsorption material and electrode

ABSTRACT

A method produces activated carbon, suitable in particular for use in double-layer condensers. The method includes a) producing a mixture of a preferably pulverulent carbon material, a base and a hydrophilic polymer chemically inert to the base, b) pressing the mixture produced in step a) to form a pressing and c) activating the pressed body produced in step b).

CROSS-REFERENCE TO RELATED APPLICATION

This is a divisional application of U.S. patent application Ser. No.13/606,641, filed Sep. 7, 2012; which was a continuation application,under 35 U.S.C. §120, of International application PCT/EP2011/053442,filed Mar. 8, 2011; the application also claims the priority, under 35U.S.C. §119, of German patent application No. DE 10 2010 002 706.5,filed Mar. 9, 2010; the prior applications are herewith incorporated byreference in their entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for producing activatedcarbon, suitable in particular for use in double-layer condensers.

Activated carbon, also known as activated charcoal, has a very highporosity, and so is used particularly in for removing of unwanted dyes,flavoring agents and/or odorizing agents from gases and liquids, duringwastewater treatment or air purification for example. In this context,the activated carbon may be used in the form of granulate, a powder orpellets depending on the application in question.

Besides this use, because of its high porosity activated carbon is alsooften used as an electrode material, for example in double-layercondensers, the use of which is becoming increasingly widespread becauseof their high energy density. Such double-layer condensers areconfigured with two electrodes, kept apart from one another by aseparator and coated with electrolyte. In order to maximize thecondenser's energy density, the electrodes used in the double-layercondensers must have the highest density possible as well as highporosity.

Activated carbon is generally produced by oxidative thermal activationat 600 to 1000° C., during which a portion of the carbon is transformedinto carbon monoxide, which in turn creates more pores in the carbon,increasing the surface area of the substance further still. In thiscontext, oxidative thermal activation usually takes place in thepresence of a strong base, preferably potassium hydroxide or anotheralkali metal hydroxide, and this is why carbon that is activated in amethod of this kind is also referred to as alkali-activated carbon orbase-activated carbon.

One problem with using alkalis or bases such as potassium hydroxide toactivate carbon consists in that a by-product of the oxidative thermaltreatment of carbon with a base is a reduction product of the base, inthe case of potassium hydroxide this by-product being metallicpotassium, which is strongly corrosive and accordingly causes corrosionof the installation in which the activation is carried out.

In order to minimize this problem of corrosion, a method for producingactivated carbon is suggested in published, European patent applicationEP 1 498 389 A1, corresponding to U.S. Pat. No. 7,709,415, in which acarbon material in powder form, for example pulverized pitch fibers, ismixed with potassium hydroxide particles. The mixture thus obtained isthen granulated at a temperature of at least 80° C. and preferably underreduced pressure to form granulate particles having an average particlediameter not exceeding 50 mm, after which the granulate particles aredehydrated at a temperature of at least 200° C., preferably underreduced pressure, and the dehydrated granulate particles are thenactivated in a nitrogen stream at a temperature between 500 and 900° C.Then, the activated carbon particles may be combined with binding agentsand filler materials as necessary and shaped into electrodes. Thisdocument also describes an alternative method, in which a mixture ofcarbon material in powder form and potassium hydroxide is subjected tothermal treatment up to a maximum of 300° C. and the mixture treated inthis way is then compacted by compression molding into pellets having adiameter not exceeding 20 mm, and these pellets then undergo heattreatment at a temperature between 600 and 1,000° C. in a nitrogenstream. Then the pellets are washed with dilute hydrochloric acid andagain with water to obtain activated carbon particles with a particlediameter of 20 μm, for example.

Although the methods described in published, European patent applicationEP 1 498 389 A1 result in less corrosion of the activation equipmentthan in corresponding methods in which activation is carried out using apotassium hydroxide fusion, corrosive potassium vapor is still producedin these methods and it is spread throughout the entire installation bythe nitrogen stream, so that the potassium also accumulates in thecooler areas of the apparatus and causes corrosion there. Apart fromthis, this method is very complicated, mainly because of the need tocreate granulate, which preferably takes place at elevated temperatureand reduced pressure.

SUMMARY OF THE INVENTION

The object of the present invention is therefore to provide a method forproducing activated carbon that is simple to perform, in which corrosionof the device in which the carbon is activated is reliably prevented,and by which activated carbon is obtained that has a density andporosity that is extremely well suited for use in double-layercondensers.

This object is solved according to the invention by a method forproducing activated carbon that contains the following steps:

-   a) producing a mixture of a carbon material, a base and a    hydrophilic polymer that is chemically inert with respect to the    base,-   b) compacting the mixture produced in step a) to form a briquette,    and-   c) activating the briquette produced in step b).

The solution is based on the surprising discovery that a method in whicha mixture is first made from a preferably pulverulent carbon material, abase and a hydrophilic polymer that is chemically inert with respect tothe base and then compacted to form a briquette before being activatedin a thermally oxidative process effectively avoids the formation anddistribution of the reduction product of the base, such as vaporizedpotassium, within the installation where the activation takes place.This is due firstly to the fact that neither during nor after theactivation is a powder used from which potassium vapor can escape easilywhile the high temperatures operating during activation prevail becauseof its large surface area to weight ratio compared with a briquette.Secondly, the addition of the hydrophilic polymer when the mixture iscompacted results in a briquette that is dense and structurally stableparticularly under the high temperature conditions that prevail duringthe activation, because the polymer was surprising found to function asa binding agent, that is to say it binds the carbon material particlesand the base particles together. For this reason, the briquette isreliably prevented from falling apart even during exposure to the hightemperatures used in the activation. Since the briquettes are so stable,internal contact between the reagents is enabled during the activation,which promotes more intense reactivity during the activation, that is tosay greater consumption of the base used, so that only a comparativelysmall amount of base needs to be used. Moreover, in the method accordingto the invention, activation does not have to take place in a gas streamsuch as a nitrogen stream; instead, the pyrolysis gases from the carbonmaterial and the hydrophilic polymer automatically result ininertization during the activation, so that the potassium vapor that ispresent cannot be spread throughout the installation. A furtheradvantage of the method according to the invention consists in that thesize of the briquette is freely selectable, rendering the methodextremely flexible. This method may also be used particularly tomanufacture very large plates, thereby enabling efficient charging ofthe furnace chamber.

For the purposes of the present invention, the carbon material used inprocess step a) of the method according to the invention is understoodto be any material that contains large quantities of carbon,particularly a material whose composition includes at least 70% byweight carbon, particularly preferably at least 80% by weight carbon,and especially preferably at least 90% by weight carbon.

Also for the purposes of the present invention, a hydrophilic polymer isunderstood to be a polymer that is liquid at 23° C. and is soluble inwater at a rate of 10 g/l at 23° C., or a polymer that is solid at 23°C. but which has a contact angle with water that is less than 90°. Forthe purposes of the present invention, the term polymer also extends tooligomers as well as polymers in the narrower sense.

For the purposes of the present invention, a polymer that is chemicallyinert with respect to the base used is understood to be a polymer thatdoes not react with the base, and particularly is not decomposed,especially does not undergo any shortening of its polymer chain afterbeing in contact with the base for 24 hours at 200° C. Moreover, thechemically inert polymer preferably does not lose any of its bindingproperties when in contact with the base for 24 hours at 200° C.

Process steps a), b) and c) are preferably carried out immediately oneafter the other, that is to say without any additional intermediatesteps, that is to say the mixture produced in process step a) and thebriquette produced in process step b) undergo process step b) andprocess step c) respectively particularly with no dehydrating and/orgranulating step in between. In this way, activated carbon may bemanufactured simply, quickly and inexpensively.

According to the invention, any hydrophilic oligomer or polymer that ischemically inert with respect to the base used may be used in processstep a). For example, good results are achieved if the hydrophilicpolymer used is a polyether, or preferably a polyetherpolyol.

In a refinement of the inventive thought, it is suggested that thehydrophilic polymer for use in process step a) may be a polyetherpolyolaccording to the following general formula (I):

HO(—R—O—)_(n)H   (I),

wherein n is a whole number between 2 and 100,000, preferably between 2and 1,000 and particularly preferably between 100 and 600, and R is astraight-chain or branched-chain alkylene group, possibly substitutedwith one or more hydroxyl group(s), preferably a C₁-C₁₅ alkylene grouppossibly substituted with one or more hydroxyl group(s), andparticularly preferably a C₁-C₁₀ alkylene group possibly substitutedwith one or more hydroxyl group(s). All of these polyetherpolyols arechemically inert with respect to standard bases and are sufficientlyhydrophilic for use in the method according to the invention.

Particularly preferred polyetherpolyols according to the general formulaI are such that have a C₁-C₆ alkylene group, possibly substituted withone or more hydroxyl group(s), as radical R, that is to say thoseselected from the group of polymethylene glycol, polyethylene glycol,polypropylene glycol, polybutylene glycol, polypentylene glycol,polyhexylene glycol, polyglycerines and any mixtures of two or more ofthe aforementioned compounds. Polyglycerines that are particularlysuitable for the present invention are compounds having the generalformula II:

wherein n is a whole number between 2 and 100,000, preferably between 2and 1,000 and particularly preferably between 100 and 600.

According to a particularly preferred embodiment of the presentinvention, polypropylene glycol and/or polyethylene glycol is used asthe hydrophilic polymer in process step a), in which case liquidpolypropylene glycol and/or polyethylene glycol, and particularlypolyethylene glycol having a weight-averaged molecular weight (Mw) from200 to 600 g/mol has proven particularly suitable. If solidpolypropylene glycol and/or polyethylene glycol is used, it ispreferably used in the form of a fine powder having an average particlediameter between 0.1 and 1,000 μm, particularly preferably an averageparticle diameter between 0.5 and 50 μm, and especially preferably anaverage particle diameter between 1 and 10 μm, so that the solidpolypropylene glycol and/or polyethylene glycol may be mixedhomogeneously with the carbon material. In accordance with the usualdefinition of this parameter, an average particle diameter is understoodto be the d₅₀ value, that is to say the particle diameter value islarger than that of 50% of the particles present, that is to say 50% ofall the particles present have a particle diameter that is smaller thanthe d₅₀ value.

Particularly in the case that a liquid hydrophilic polymer is used inprocess step a), in order to prevent the base from dissolving in thepolymer it is preferred to mix the hydrophilic polymer with the carbonmaterial first, before adding the base to the mixture that is producedin this manner and mixing it therewith. The mixer that is preferablyused for this process is an intensive mixer.

In general, all bases that are suitable for oxidative chemicalactivation of carbon may be used in process step a), although alkalimetal hydroxides and alkali metal carbonates are particularly suitablefor this purpose, such as preferably lithium hydroxide, sodiumhydroxide, sodium carbonate and potassium carbonate, and especiallypreferably potassium hydroxide.

If, as is preferred, the base is a solid at room temperature, it too ispreferably added in powder form, wherein the average particle diameterof the base is preferably between 0.1 and 1,000 μm, and particularlypreferably between 0.5 and 100 μm.

In principle, all carbon materials are usable in process step a), andgood results are obtained particularly with the products of pyrolysisfrom coal, mineral oil or wood, such as pitch and coke. Especially goodresults are obtained when green coke is used as the carbon material inprocess step a) that is to say non-calcined coke containing 10 to 15%volatile components. In this context, all types of green coke lendthemselves to use in the present invention, including for exampleisotropic coke, electrode coke and needle coke, and especiallypreferably pulverulent green coke having an average particle sizebetween 0.1 and 1,000 μm. The actually preferred particle diameter ofthe green coke used in process step a) depends on the nature of theapplication in which the activated carbon will be used subsequently. Forexample, whereas average particle diameters of about 500 μm arepreferred if it is to be used as adsorption material, if the activatedcarbon is to be used as electrode material a smaller average particlediameter is preferred, in particular an average particle diameterbetween 0.5 und 50 μm, and especially an average particle diameterbetween 1 und 10 μm. If the activated carbon is to be used in adouble-layer condenser, the average particle diameter of the green cokeused in process step a) is preferably not larger than 5 to 10 μm.

In the context of the present invention, it has also proven advantageousif the carbon material used in process step a), preferably green coke inpowder form, has very little or no porosity smaller than 10 m²/g.

In general, the individual components in process step a) may be used inany proportion relative to each other, and the degree of activation ofthe carbon is adjusted with the relative quantity of the base, with theproviso that a higher base content in the mixture produced in processstep a) also determines a larger specific surface area of the activatedcarbon in the mixture produced in process step a), and the structuralstability of the briquette produced in process step b) is adjusted usingthe content of hydrophilic polymer, with the proviso that a higherpolymer content determines greater structural stability of thebriquette. For this reason it is preferred that the hydrophilic polymeris present in the mixture in a proportion of 3 to 10% by weight, whereasthe relative proportion of carbon to base is preferably 1:1.5 to 1:2.

Based on these trends, in a refinement of the inventive thought it issuggested to produce a mixture in process step a) that contains 20 to50% by weight carbon material, 1 to 15% by weight hydrophilic polymerand 35 to 79% by weight base, preferably 25 to 40% by weight carbonmaterial, 2 to 10% by weight hydrophilic polymer and 50 to 73% by weightbase, and particularly preferably 30 to 35% by weight carbon material, 3to 7% by weight hydrophilic polymer and 58 to 67% by weight base.

In a particularly preferred embodiment of the present invention, themixture produced in process step a) contains 25 to 40% by weight greencoke, 2 to 10% by weight polyethylene glycol having a Mw of 200 to 600g/mol, and 50 to 73% by weight potassium hydroxide, and particularlypreferably 30 to 35% by weight green coke, 3 to 7% by weightpolyethylene glycol having a Mw of 200 to 600 g/mol and 58 to 67% byweight potassium hydroxide. In these proportions, it is possible toobtain activate carbon with a BET surface area of up to 1,500 or even2,000 m²/g by the method according to the invention.

In process step b), the mixture produced in process step a) is compactedin accordance with the invention to form a briquette. For the purposesof the present invention, a briquette is understood to be a compactedbody that has a longest dimension, that is to say in the case of an atleast essentially spherical briquette, a diameter, or in the case of apolygon a length, of at least 50 mm, preferably at least 100 mm,particularly preferably at least 1 cm, and most preferably at least 10cm. An example of such is a cuboid briquette having both length andwidth of about 50 cm.

In general, the compacting in process step b) may be performed in anysuitable compression mold, wherein the density of the briquette isincreased as the compacting pressure rises, and consequently the maximumfurnace charge during activation is increased. This is why compacting inprocess step b) is preferably carried out in such manner that themixture produced in process step a) is compacted to form a briquettehaving a density of at least 1 g/cm³, preferably a density of at least1.25 g/cm³, particularly preferably a density of at least 1.5 g/cm³, andmost preferably a density of at least 1.7 g/cm³.

For example, a briquette having a density of about 1 g/cm³ may beproduced with a compacting pressure of 100 kg/cm², whereas briquetteshaving a density of about 1.7 g/cm³ require a compacting pressure of 5tons/cm². For this reason, the compacting in process step b) ispreferably carried out in a die press with a pressure of at least 100kg/cm².

The most important factors for successful heat treatment according toprocess step c) are the maximum temperature reached during the heattreatment and the hold time for which this maximum temperature ismaintained. According to the invention, the heat treatment of thebriquette in process step a) is carried out at a maximum temperature of500 to 1,500° C., this being set preferably to 700 to 1,000° C.,particularly preferably to 700 to 900° C., and most preferably to 850 to900° C.

In this context, it is preferred that the maximum temperature ismaintained for at least 0.5 hours, particularly preferably for at least1 hour, especially preferably for at least 2 hours, and most especiallypreferably for at least 3 hours.

The preferred heating rate depends on the quantity of material in thefurnace, wherein it is recommended to use lower heating rates forheating large quantities of material uniformly than are required to heatsmaller material quantities uniformly. Depending on the quantity ofmaterial, good results are generally obtained in the furnace if theheating rate is 1 to 100° C./min, preferably 2 to 50° C./min andparticularly preferably 5 to 25° C./min.

In a refinement of the inventive thought, it is suggested to cool thebriquette rapidly to room temperature after maintenance of the maximumtemperature in process step c), and this may be carried outadvantageously in such manner that the briquette is cooled to about 150°C. in the furnace, before the briquette is quenched, preferably inwater.

According to another preferred embodiment of the present invention, theactivated briquette is washed in a process step d) following the heattreatment in order to remove impurities from the activated carbon. Inthis context, the washing procedure preferably includes at least awashing step with a mineral acid, such as hydrogen chloride or sulphuricacid, followed by repeated washing with distilled water until neutralityis reached.

A further object of the present invention is activated carbon that isobtainable with the method described in the preceding.

Such an activated carbon preferably has a BET surface area of 1,500 to2,000 m²/g.

A further object of the present invention is the use of the activatedcarbon described in the preceding as an adsorption material or as anelectrode, and preferably as an electrode in a double-layer condenser.

Other features which are considered as characteristic for the inventionare set forth in the appended claims.

Although the invention is described herein as embodied in a method forproducing base-activated carbon, it is nevertheless not intended to belimited to the details described, since various modifications andstructural changes may be made therein without departing from the spiritof the invention and within the scope and range of equivalents of theclaims.

The construction and method of operation of the invention, however,together with additional objects and advantages thereof will be bestunderstood from the following description of specific embodiments whenread in connection with the accompanying example.

DETAILED DESCRIPTION OF THE INVENTION

1,666 g green coke (isotropic pitch coke) from the company Sasol SynfuelLtd. and having an average particle diameter of 3 μm and 166 gpolyethylene glycol with a Mw of 200 g/mol were introduced into anintensive mixer manufactured by Eirich and mixed together for 10minutes. Then, 3,166 g potassium hydroxide having an average particlediameter of approximately 10 μm was added to this mixture and mixingcontinued for a further 15 minutes. At the end of the mixing operation,a homogeneous mixture having an average particle diameter of about 3 μmwas obtained.

The mixture was then introduced into a press manufactured by companyWickert, model WKP2000S, with a die size of 50 cm×50 cm×10 cm and wascompacted at room temperature under pressure of 95 kg/cm² to form abriquette having a density of 1.0 g/cm³.

The briquette was then placed in a furnace and subjected to heattreatment with a heating rate of 5° C./min., a maximum temperature of850° C., and a holding time of 1 hour before the activated briquette wasquenched in water and then washed with a mineral acid, in this casesulphuric acid, followed by repeated washing operations with distilledwater until neutrality was reached.

The activated carbon produced in this way was shaped to make electrodesin the form of round briquettes, each having a diameter of 10 mm and amass of approximately 10 mg, and which were measured in a Swagelok cellwith 1 M tetraethyl ammonium tetrafluoroborate in acetonitrile as theelectrolyte with a “Whatman” glass fiber separator having a thickness of30 μm. The double-layer condenser reached a specific electricalcapacitance of 146 F/g relative to the electrode at 2.3 V and a chargingcurrent of 500 mA/g.

1-4. (canceled)
 5. A method for producing activated carbon comprising:producing a mixture of a carbon material, a base, and a hydrophilicpolymer being chemically inert with respect to the base; compacting theresulting mixture to form a briquette; and activating the resultingbriquette, wherein the hydrophilic polymer is polyethylene glycol havinga weight-averaged molecular weight (Mw) from 200 to 600 g/mol.